Привет. :)
( ∑ k = 1 n ( n x − 1 ) = n x ) = ( ∏ k = 1 x ( n ) = n x ) {\displaystyle (\sum _{k=1}^{n}{(n^{x-1})}=n^{x})=(\prod _{k=1}^{x}{(n)}=n^{x})}
